1. Field of the Invention
This invention relates to methods and tools for removing a plug from a conduit, and, more particularly, to the removal of a plug from a heat exchanger tube mounted in a tube sheet of a nuclear powered steam generator by simultaneously applying a static pull force and an impulsive and repetitive, or periodic, pull force to the plug.
2. Description of the Prior Art
Tooling systems for removing plugs from heat exchanger tubes in nuclear powered steam generators are known in the prior art. It is desirable to remove such plugs in order that marginally corroded or damaged heat exchange tubes can be refurbished and put back in service, thereby increasing the capacity of a nuclear steam generator to produce power.
The plugs typically used for plugging heat exchanger tubes in nuclear steam generators are formed from a tubular shell fabricated of a nickel alloy, such as Inconel.RTM.. The shell is open at an end near an open end of the conduit, or tube, near a face of a tube sheet in the steam generator, and closed at its opposite end distal from the tube opening. One common type of plug, described in commonly owned U.S. Pat. No. 4,390,042, to Kucherer, includes an internally threaded plug skirt at the open end and a tapered, cork-shaped expander member contained completely in the interior of the shell. Before fixing the plug to the tube, the larger, circular end of the expander member is in abutment with the inner surface of the closed end of the plug shell. The shell inner surface is slightly tapered from the closed end to an axial position near the threaded plug skirt. When the cork-shaped expander member is forcefully drawn from the closed end towards the open end of the shell by a hydraulic ram, it radially expands the plug into sealing engagement with the interior surface of the tube by a wedging action. The forceful pulling of the cork-shaped expander member along the longitudinal axis of the shell further applies an extruding force to the metallic walls of the shell. A plurality of annular lands circumscribing the outer walls of the shell become sealingly engaged against the interior surface of the heat exchanger tube.
The traditional method for removing mechanical plugs, such as the plug described above, includes the steps of first pushing back the internal expander with a push-rod, and then pulling the plug with a hydraulic puller. Typically, the threads of the plug skirt are used for attachment of the puller to the plug. This method of pushing the expander back and pulling has severe problems. First, and most limiting, is that the bottom of the plug (the plug skirt) often breaks off before the plug dislodges. This is particularly undesirable since the only remaining recovery method, drilling, is now made difficult by a loose expander. When a plug is pulled successfully, the inner surface of the tube is often deeply scored by the pulling process. This scoring usually dictates a reaming process if the tube is to be replugged at a later date and may be severe enough to prevent a tube from being put back in service.
Another method for removing such plugs is described in commonly owned U.S. Pat. No. 4,903,392, to Stickel, et al. According to this method, the plug is heated with an electrically conductive push-rod by ohmic heating to a temperature that lowers its tensile yield strength, and an axial force is applied to the plug to elongate it, thereby radially contracting the plug and relaxing the engagement between the plug and the inner surface of the tube. After cooling, the plug may then be pulled out of the tube. This heat relaxation method requires a complex tooling system and also has some severe limitations. The plug shell can tear, or separate, during the step of pushing back the expander or elongating the plug shell. In this event, the tube will most likely need to be reamed and replugged. U.S. Pat. No. 4,800,637, to Overbay, also describes a method by which the plug shell is mechanically elongated, but without the heating step.
Another method for removing such plugs from heat exchanger tubes is disclosed in commonly owned U.S. Pat. No. 4,829,660, to Everett et al. The expander element is pushed back out of engagement with the tapered inner surface of the plug and forced through the closed end of the plug shell. A TIG torch is then used to create beads along the longitudinal axis of the plug shell to relax the engagement between the shell and the tube. This method also requires a complex tooling system. Further, if the plug shell separates during the expander push-back, a TIG burn will be made in the tube wall, likely requiring reaming and replugging of the tube.
Plug drilling, such as disclosed in commonly owned U.S. Pat. No. 4,734,972, to Hawkins, is another method of plug removal but is usually undesirable because of the complexity of the process and the risk of damaging the tube sheet and the tubes. Drilling is also very slow and impractical for large numbers of plugs.
Alternative methods of plug removal have not been forthcoming because of the limitation of the breakage of the bottom of the plug and the difficulty in finding an alternate place to apply a removal force. Therefore, there is a need for a new method for removing plugs from heat exchanger tubes and similar conduits that does not suffer the disadvantages of the prior art methods.